We have created a simple reporter system for the presence of Cre recombinase. A strong promoter with widespread activity normally drives the expression of a red fluorescent protein, but Cre recombinase can catalyze a recombination event that replaces the red fluorescent protein with a green one. We have tested this system in cell lines and now propose to test it in transgenic lines of mice. Specifically, we will screen founders for the presence of red fluorescent signal and then cross these with a line that expresses Cre recombinase in endothelial cells to determine whether the recombination even can occur at the genomic level in a transgenic mouse line. In addition, we have will modify the reporter system by replacing the DsRed sequence with two other sequences that may improve the utility of this reporter. These are straightforward goals that should only take a year to accomplish and the potential benefit is the production of reagents and mouse lines that may be quite useful for many vision scientists.